1. Field of the Invention
The invention, in general, relates to a novel method of determining the quanity of solid or liquid particulate matter suspended in a gaseous carrier medium of a two-phase flow and, more particularly, to a method of determining the quanity and quantitative distribution of pulverized coal in a gaseous carrier medium of a pneumatic feed pipe system.
2. The State of the Art
The transportation of minute particles of solid materials or droplets of liquid, hereinafter sometimes referred to a particulate matter, in a gaseous carrier medium of a two-phase flow often necessitates turbulent flows to ensure a sufficient transport velocity and a sufficient quantity of the particulate matter. In transporting particulate matter, in particular pulverized coal, through burner feed pipes of coal-burning power stations, for instance, it has been practically impossible to prevent the formation of so-called ropes. These are sections in the flow pattern of increased density of the particulate matter, i.e., pulverized coal. Ropes may be geometrically stable and localized; but they may also occur stochastically at different positions, and they may change their dimensions and density and migrate within a feed pipe. At splitter boxes in particular, ropes are likely to cause highly irregular distributions of particulate matter and therefore lead to significant differences in the quantities of particulate matter transported within individual feed pipes. It has not only been very difficult to determine the quantity or flow rate of particulate matter transported in pneumatic feed systems, in particular multiple-branch ones, but also to ensure a uniform quantitative cross-sectional distribution of such matter within feed pipes of the kind here under consideration.
Known systems, such as those utilizing tubes for isokinetically withdrawing samples, for instance, often yield strongly distorted results, for they can only detect ropes by chance or, more likely, not at all. Furthermore, such measurements are extremely time-consuming, and as a rule may take hours to deliver results. They are, therefore, by and large useless for closed-loop controls.
For the control of pneumatic feeder systems and for regulating the flow of particulate matter through multiple-branch systems, quickly responding measuring systems are of the utmost importance. For that reason, various attempts have been made to utilize microwaves for such measurements. In such systems, microwaves of predetermined frequencies are fed into an elongate section of a pipe set up as a measuring path, and at the end of the measuring path any change in amplitude and phase of the microwave is registered. The principle underlying measurements with microwaves is that charging or loading a carrier gas with pulverized solids and/or aerosolized liquids changes the complex dielectric constant within the feed pipe and that the microwaves are subject to attenuation and phase shifting as a function of the dielectric constant.
Such methods have been generally disclosed, for instance, by European Patent 0,717,269 and U.S. Pat. No. 5,177,444. However, for pneumatic feed systems, neither sensitivity nor precision of known methods utilizing microwaves are sufficient, especially where the flow rates of a given material differ in individual pipes or pipe sections of multiple-branch feed pipes, or where significant differences in local or homogeneous distribution and volumetric concentration of the particulate matter are encountered within a pipe system in consequence of the mentioned rope formation.
U.S. Pat. No. 4,423,623 describes a microwave measurement system for measuring coal slurries. In coal slurries, microwaves are subject to strong attenuations, and the sensitivity required for measuring coal in water is orders of magnitude lower than the sensitivity required for measuring pulverized coal in a gaseous carrier medium. Ideally, an adequately precise regulation of pulverized coal fed to the burner system of a power plant boiler does not only require a measuring sensitivity or responsiveness in the order of 1 g of coal dust per cubic meter of carrier gas but also a response time in the millisecond range. However, such small load deviations result in extremely small changes of the complex dielectric constant, and their effect on the attenuation and phase of a microwave is very small indeed.
Moreover, the use of microwaves for measuring loads in pneumatic transport systems leads to significant problems because of disturbances caused by reflected microwaves. The attenuation of microwaves is particularly small at low loads, so that analogous to waveguides they propagate in a pipe system over large distances and are reflected by constrictions, splitter boxes, ends of pipes and the like. This may lead to superposed waves propagating to and fro and, consequently, to significant distortions of the results of measurements.
The known methods utilizing microwaves suffer from the further disadvantage of requiring considerable and complex equipment. Usually, an existing feeder pipe system has to be fitted with a pipe section of highly precise geometry to provide a measuring path as well as with complementary transmitting and receiving antennae. Other known methods include the complex mounting of slotted couplers as transmitting and receiving devices in existing pipe sections satisfying predetermined geometric requirements.